If no TTY serial (matching the given filters, if any) was found, use
the exit code `1`. The idea is that simple shell scripts falling back
to alternative variants of a board can be used via

```.sh
ttys.py --most-recent --model Fooboard --vendor Footronic || \
    ttys.py --most-recent --model Barboard --vendor Bartronic
```

Just adding a regex that would accept both vendors and models would
have different semantics: If both a Fooboard and a Barboard are
attached, it would pick the most recently connected of both. The shell
expression above would always prefer a Fooboard over a Borboard.

The use case cheap Arduino clones that replace the ATmega16U2 used
as USB UART bridge with cheap single purpose chips. The original
ATmega16U2 has the advantage that it provides identification data
unique the specific Arduino board, while the clones cannot be told
apart from standalone USB UART bridges or Arduino clones of other
models. Hence, we want to pick the genuine Arduino board if connected,
and only fall back to matching cheap USB UART bridges if no genuine
Arduino board is connected.

Allow overriding the shell command used to auto-detect the TTY of a
board with `MOST_RECENT_PORT=1` via the `TTY_SELECT_CMD` variable.
The use case is to also detect Arduino Mega 2560 clones with cheap
USB UART bridges (for which the filter command may yield false
positives) while preferring genuine Arduino Mega 2560 boards (if
found) over the clones (as the filter for genuine boards does not yield
false positives).

This allows automatically selecting TTY actually belonging to an
Arduino Mega2560 if `MOST_RECENT_PORT=1` is set.

Note: Unless `ARDUINO_MEGA2560_COMPAT_WITH_CLONES` is set to `0`,
this will fall back to detecting any cheap USB UART bridge typically
found in Arduino Mega 2560 clones if no genuine Arduino Mega is found.